The present invention relates to a method of evacuating treating containers to a vacuum for use in compacting radioactive wastes by an HIP (hot isostatic press), hot press or the like. Such radioactive wastes include metals and bricks contaminated with plutonium or a transuranium element having a long half life.
In recent years, attention has been directed to treatments by HIP or the like for compacting radioactive wastes into solid blocks for stabilization before storing the wastes (see, for example, Examined Japanese Patent Publication SHO 57-959).
For example, the treatment of hulls will be described which are sheared cladding tubes. Hulls are hollow, have a low bulk density of 1.0 and are therefore precompressed to a true density ratio of at least about 70% by a press first. During the compression, a highly radioactive oxide formed by zircalloy on the surfaces of the hulls and having a thickness of about 10 .mu.m partly separates off.
The compressed waste is then placed into a treating container of stainless steel or the like, which is then filled with a metal powder, stainless steel powder or the like to eliminate the space or clearances remaining in the container. A closure is then welded to the container, piping (hereinafter referred to as an "evacuating pipe") is thereafter attached to the closure for connection to a vacuum pump, and the interior of the container is evacuated to a degree of vacuum, e.g., about 10.sup.-2 torr. The container thus evacuated is completely sealed off to hold the vacuum therein, and the container is compressed by HIP or hot press under an external pressure with heating, whereby the container is compacted. The container is evacuated to prevent the container itself from breaking owing to the presence of air or like gas confined in the container when the container is compressed under a high pressure.
However, if the container is thus evacuated after the waste has been placed thereinto, a particulate radioactive substance separating off the waste is led out of the container via the evacuating pipe to contaminate the vacuum pump and the inner surface of the evacuating pipe. The spillage of the radioactive substance due to aspiration can not be prevented completely even at a reduced evacuation rate. Further even if a filter is removably installed in the evacuating pipe and the like, the filter becomes contaminated and is therefore extremely difficult to replace, hence inconvenience is caused.